improvements for the use prompt command

[legomushroom]

Improves the use prompt command:

• updates command ID to avoid collisions
• includes currently open prompt file into selection list (if file is not included by other means, like the prompts location setting)
• adds cmd + arrow gesture support to the prompts selection dialog
• refactors to improve the file structure

Part of https://github.com/microsoft/vscode-copilot/issues/13324.